A membrane protease is targeted to the relict plastid of Toxoplasma via an internal signal sequence

The apicoplast is a secondary plastid found in Toxoplasma gondii, Plasmodium species and many other apicomplexan parasites. Although the apicoplast is essential to parasite survival, little is known about the protein constituents of the four membranes surrounding the organelle. Luminal proteins are directed to the endoplasmic reticulum (ER) by an N-terminal signal sequence and from there to the apicoplast by a transit peptide domain. We have identified a membrane-associated AAA protease in T. gondii, FtsH1. Although the protein lacks a canonical bipartite-targeting sequence, epitope-tagged FtsH1 colocalizes with the recently identified apicoplast membrane marker APT1 and immunoelectron microscopy confirms the residence of FtsH1 on plastid membranes. Trafficking appears to occur via the ER because deletion mutants lacking the peptidase domain are retained in the ER. When extended to include the peptidase domain, the protein trafficks properly. The transmembrane domain is required for localization of the full-length protein to the apicoplast and a truncation mutant to the ER. Thus, at least two distinct regions of FtsH1 are required for proper trafficking, but they differ from those of luminal proteins and would not be detected by the algorithms currently used to identify apicoplast proteins.